This invention relates to a machining device such as a wire cut electric discharge machine which has a machining solution jetting nozzle.
FIG. 5 shows the positional relationship between an X-Y cross table and a machining head in a wire cut electric discharge machine. In FIG. 5, reference numerals 1 designates a workpiece to be machined which is placed on the X-Y cross table 2 which is movable in a horizontal plane; and 3, the machining head which has a machining solution jetting nozzle 4 and an electrode guide 6 and is fixedly secured to a shaft 7 which is movable in a Z-axis direction in the electric discharge machine body (not shown). The X-Y table 2 and the shaft 7 are moved by drive motors 15 and 16, and a drive motor 17, respectively, and the amounts of movement of them are controlled by a control unit 9. By moving the X-Y cross table 2, the workpiece 1 and the wire electrode 5 are two-dimensionally moved relative to each other. The workpiece 1 and the nozzle 4 are positioned in place by moving the machining head 3. In the case where the workpiece 1 is not uniform in thickness, the machining head 3 is automatically moved in the Z- axis direction according to the thickness.
In order to increase a machining speed in a wire cut electric discharge machining operation, it is necessary to efficiently and sufficiently remove sludge from the machining groove. For this purpose, it is essential to minimize the amount of machining solution leaking through the gap between the workpiece 1 and the mouth of the machining solution jetting nozzle 4, thereby to increase the pressure of the machining solution in the machining groove. In general, the distance between the mount of the machining solution jetting nozzle 4 and the workpiece 1 is about 100 .mu.m; that is, it is rather difficult to visually confirm. The machining head 3 must be in the Z-axis direction according to the thickness of a workpiece 1 while the distance being maintained therebetween until it is accurately positioned in place. For this purpose, the following two methods of positioning the machining head may be considered: In the first method, the machining head is positioned with a thickness gage or the like interposed between the workpiece 1 and the end face of the nozzle 4. In the second method, after the end face of the nozzle 4 is brought into contact with the workpiece 1, the machining head 3 is lifted by a required distance. Therefore, in order to prevent the difficulty that, while the machining head 3 is being moved with the drive motor so as to be positioned, the machining solution jetting nozzle 4 is broken colliding with the workpiece in error, it is necessary to provide means for detecting the collision to stop the machining head.
FIGS. 6A and 6B are a sectional view showing a conventional machining head. The machining solution jetting nozzle 4 is secured to the lower end portion of the machining head through a spring 10 with a retainer 11. The machining head has a limit switch 12 to detect the lift of the machining solution jetting nozzle 4. The output of the limit switch 12 is applied to the control unit 9.
In the FIG. 6A, the nozzle 4 is not in contact with the workpiece 1. Under this condition, the nozzle 4 is so held by the spring 10 and the retainer 4 that the limit switch 12 is not operated. As the machining head 3 is moved towards the workpiece1, the nozzle 4 is brought into contact with the workpiece 1, and stopped thereby. When, under this condition, the machining head 3 is further moved towards the workpiece 1, the limit switch 12 secured to the maching head 3 is struck against the upper end face of the machining solution jetting nozzle 4 so that it is operated to output a signal. The signal is applied to the control unit 9 to stop the movement of the machining head 3.
As was described above, in the conventional wire cut electric discharge machine, when the machining head is moved downwardly, in the Z-axis direction, its collision with the workpiece is detected as follows: After the machining solution jetting nozzle of the machining head has been brought into contact with the workpiece, the limit switch is mechanically operated to output the signal to stop the movement of the machining head. However, the method is disadvantageous in that the limit switch may be not correctly operated because of the sludge accumulated during machining so that the machining head is not stopped, with the result that the machining solution jetting nozzle, the shaft, and the machining head are broken. Furthermore, in the above-described conventional wire cut electric discharge machine, it cannot be detected when the workpiece is moved horizontally to collide with the nozzle.